1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same and, more particularly, a semiconductor device having quantum dots and a method of manufacturing the same.
2. Description of the Prior Art
With the progress of semiconductor process, the film forming technology and the fine pattern technology in nano scale are going to be employed to form the semiconductor device. As such film forming technology and such fine pattern technology make progress, the integration density of the semiconductor integrated circuit can be improved and also the devices utilizing the quantum-mechanical effect, e.g., HBT (Hetero Bipolar Transistor), the quantum well laser, etc. are put to practical use. In addition, of next generation devices which employ new material are studied in recent years. For example, the quantum dot memory which employs the hole burning effect has been proposed, in Shunichi Muto, Jpn. J. Appl. Phys. Vol.34 (1995) pp. L210-L212.
In recent years, the quantum dot is observed with interest as ultimate structure utilizing the quantum-mechanical effect. The quantum dot is an extremely fine potential box in which quantum confinement of the carriers occurs three-dimensionally. The quantum dot has the state density like a delta function. And, only two carries enter into the ground level of one quantum dot.
As one of the devices utilizing such characteristic of the quantum dot, it is proposed to employ the quantum dot in the active region of the semiconductor laser.
In the semiconductor laser having the quantum well structure, the limits of improvement of the oscillation threshold current and the temperature characteristic of the threshold current are pointed out. However, efficiency in mutual action between the electron/hole and the light can be increased up to the utmost limits by applying the quantum dot to the active layer, and thus the oscillation threshold current and the temperature characteristic of the threshold current can be improved.
In addition, the blue chirp modulator, the wavelength converting device, the single electron transistor, or the quantum dot memory utilizing the hole burning effect has been proposed. The formation of next generation devices by utilizing the quantum dot are studied energetically.
As the technology for forming such quantum dot, the fine pattern technology is employed. For example, the lithography method using the electron beam, the forming method on the bottom of the tetrahedral hole, the method of utilizing the lateral growth on the finely inclined substrate, or the atomic manipulation method utilizing the STM (Scanning Tunneling Microscope) technology has been proposed. The structure of the quantum dot formed at a vertex of the pyramidal crystal is written in U.S. Pat. No. 5,313,484, and the method of forming the quantum dot on the inner surface of the tetrahedral hole is written in U.S. Pat. No. 5,656,821.
Since those methods have a common feature to work artificially, they have such an advantage that the quantum dot position can be controlled arbitrarily. However, the number density of the quantum dots cannot exceed the accuracy limit of the fine pattern technology and also the uniformity of the quantum dots is extremely low.
As the new technology serving as the break-through to form the quantum dot, the technology for self-forming the quantum dot has been found recently. This technology utilizes the phenomenon that the three-dimensional fine structure (quantum dot) can be self-formed by vapor-epitaxial-growing the semiconductor having the lattice mismatching under certain conditions. This method is extremely easy to perform rather than the fine patterning. In addition, the resultant quantum dots can have the very high uniformity beyond the accuracy limit of the artificial work technology, and have the high number density and the high quality.
Such technology is described in Istavan Daruka et al., PHYSICAL REVIEW LETTERS, Vol.79, No.19, 10 November 1997. Devices such as the semiconductor laser, for example, using such self-formed quantum dot are actually reported and a possibility of the quantum dot device becomes practical.
Several forming modes in the self-formation of the quantum dot have been known. The best-known forming mode is,a mode called the Stranski-Krastanov mode (referred to as an xe2x80x9cS-K modexe2x80x9d hereinafter). In this mode, the semiconductor crystal which is epitaxially grown is grown two-dimensionally at the beginning of the growth but grown three-dimensionally at the stage beyond the elastic limit of the film. This mode can be most easily achieved in the self-forming modes and thus employed normally. According to this mode, the quantum dots can be formed at the high number density.
FIG. 1 shows the situation that InAs dots 102 which are self-formed on a GaAs substrate 101 are covered with a GaAs layer 103.
In addition, a mode called the Volmer-Webber mode is known as another mode. In this mode, the semiconductor crystal is grown three-dimensionally from the beginning without the initial two-dimensional growth. It is said that normally this mode occurs at the lower temperature than the S-K mode. However, it is hard to form the dots with high quality and therefore the study of this mode is not actually conducted.
Furthermore, as the new dot forming method utilizing the self-forming mode, a closely stacking. method attracts the skilled person""s attention. The closely stacking method is such a method that the big height quantum dots can be formed by laminating the three-dimensional structures, which are formed by the already-mentioned method, via the intermediate layer having a small thickness, through which the carriers are tunneled, along the growing direction to be put together as a lump respectively. According to this method, the quantum dots with the high uniformity can be formed.
In this manner, various methods have been found for the technology for forming the quantum dots. However, if the application of the quantum dot to the devices is considered, it is indispensable to control the energy of the quantum dots.
For example, if the case where the semiconductor laser using the quantum dots is applied to the laser light source for the optical communication is considered, the semiconductor laser whose emission wavelength is 1.3 xcexcm (0.95 eV) or 1.55 xcexcm (0.8 eV) must be formed. However, if the InAs or InGaAs quantum dots are formed on the GaAs substrate, the bandgap energy is about 1.1 to 1.3 eV. As a result, it is impossible to employ such quantum dots in the optical communication.
Moreover, in the case of the quantum dots formed by the closely stacking method, similarly the bandgap energy is about 1.1 to 1.3 eV if the InAs or InGaAs quantum dots are formed on the GaAs substrate, like the case of the single S-K mode. As a result, it is impossible to employ such quantum dots in the optical communication.
Still other subjects for the practical use of the device become apparent. There are the temperature dependency of the quantum dot energy as one of such subjects. Normally, the energy is reduced when the temperature is increased, and such temperature change affects the device characteristic. For example, if the low temperature state shown in FIG. 2A and the high temperature state shown in FIG. 2B are compared with each other, the crystal lattice strains between the quantum dots 2 and peripheral crystals 1, 3 become different.
The reason for the temperature change of the energy is intrinsic. This is because the lattice constant of the semiconductor crystal depends on the temperature and thus the bandgap is changed according to the change of the lattice constant.
That is, such phenomenon occurs in not only the quantum dot but also the quantum well. In order to overcome such phenomenon, search of new material system is carried on, but such search has not come up to the success yet.
It is an object of the present invention to provide a semiconductor device in which an emission wavelength of quantum dot can be controlled and a method of manufacturing the same.
It is another object of the present invention to provide a semiconductor device having a structure in which energy of the quantum dot is difficult to be affected by the temperature change.
According to the present invention, the quantum dots are formed on the compound semiconductor substrate by controlling the composition of the compound semiconductor substrate which contains at least three elements. Therefore, the emission wavelength of the quantum dots can be adjusted by the lattice constant of the compound semiconductor substrate. As a result, the emission wavelength of the quantum dots can be shifted to the longer wavelength side, and also the quantum dots having the emission wavelength of 1.3 xcexcm band or 1.55 xcexcm band, which is difficult to accomplish by the quantum dots formed on the GaAs substrate in the prior art, can be formed.
Further, according to the present-invention, the buffer layer whose lattice constant in the neighborhood of the surface is different from the lattice constant in the neighborhood of the interface between the buffer layer and the semiconductor substrate is formed on the semiconductor substrate, and then the quantum dots are formed on the buffer layer. Therefore, the emission wavelength of the quantum dots can be adjusted by the lattice constant in the neighborhood of the surface of the buffer layer. As a result, the emission wavelength of the quantum dots can be shifted to the longer wavelength side, and also the quantum dots having the emission wavelength in the 1.3 xcexcm band or the 1.55 xcexcm band, which is difficult to be achieved by the quantum dots formed on the GaAs substrate in the prior art, can be formed.
Furthermore, according to the present invention, since lattice strains of the quantum dots and the second semiconductor crystal layer covering the quantum dots can be relaxed by covering a part of the quantum dots with the first semiconductor crystal layer, influence of the lattice distortion upon the original energy of the quantum dots can be reduced. Therefore, an amount of change in the lattice distortion energy of the quantum dots due to the temperature change can be reduced. As a result, an amount of change in the total energy of the quantum dots can be suppressed rather than the prior art.
This is because change in the total energy of the quantum dots due to the temperature change becomes equal to a sum of the energy change as a bulk and the energy change due to the lattice distortion at a rough estimate.